Both the state and a police informer may have an interest in keeping confidential the identity of the informer. But such confidentiality is subject to a defendant's right to a fair trial. A defendant may make his own inquiries into the identity of an informer, but this must be done with care to avoid conduct that obstructs the course of justice.
The line may be crossed, for example, if the defendant tries to obstruct prosecution proceedings by suggesting that he will disclose an informer's identity. In R v Barros
2011 SCC 51 (26 October 2011) that was alleged.
It is in the public interest that false claims of informer privilege are disallowed. The person may have been a participant in the offending to an extent that protection of confidentiality is not warranted. He may have instigated the offending. Both the defendant, and when the case gets to trial, the judge, have an interest in ensuring that claims to confidentiality and privilege are properly based.
Where there is a proper basis for informer privilege it is granted - without the balancing of competing interests that may be undertaken in other areas such as confidentiality relating to journalists' sources – but only if the defendant's right to a fair trial would not be compromised [33-35].
The Canadian approach seems consistent with that in New Zealand: Evidence Act 2006, ss 53, 64, 67.